Method for forming a chemical microreactor

ABSTRACT

Disclosed is a chemical microreactor that provides a means to generate hydrogen fuel from liquid sources such as ammonia, methanol, and butane through steam reforming processes when mixed with an appropriate amount of water. The microreactor contains capillary microchannels with integrated resistive heaters to facilitate the occurrence of catalytic steam reforming reactions. Two distinct embodiment styles are discussed. One embodiment style employs a packed catalyst capillary microchannel and at least one porous membrane. Another embodiment style employs a porous membrane with a large surface area or a porous membrane support structure containing a plurality of porous membranes having a large surface area in the aggregate, i.e., greater than about 1 m 2 /cm 3 . Various methods to form packed catalyst capillary microchannels, porous membranes and porous membrane support structures are also disclosed.

RELATED APPLICATION

This application is a Divisional of U.S. patent application Ser. No.10/007,412 filed Dec. 5, 2001 now U.S. Pat. No. 6,960,235 and claimspriority thereof.

The United States Government has rights in this invention pursuant toContract No. W-7405-ENG-48 between the United States Department ofEnergy and The University of California for the operation of LawrenceLivermore National Laboratory.

BACKGROUND OF THE INVENTION

Porous membrane reactors typically utilize a bulk porous media which isaffixed to the end of stainless steel tubing through which the chemicalspecies is delivered. For the application of steam reforming hydrogencontaining fuels, a catalyst is introduced to the porous membrane andthe entire fixture is heated as gas is delivered to the membrane. Whilesteam reforming of methanol has been reported at 350° C., typicaloperating temperatures are high, e.g., 500° C. to 700° C. due to theinability of the reactor to adequately exchange heat with the outsideenvironment.

German patent application, DE 1998-19825102 discloses a method toproduce catalytic microreactors that includes “placing a catalyst in thereaction spaces.” The microreactors can be used for steam reforming orpartial oxidation of hydrocarbons to produce hydrogen gas for fuelcells.

Srinivasan et al disclose in the American Institute of ChemicalEngineers (AIChE) Journal (1997), 43(11), 3059-3069, a silicon-basedmicrofabrication of a chemical reactor (microreactor) havingsubmillimeter flow channels with integrated heaters, and flow andtemperature sensors. The article discusses the potential applications ofthis reactor and the feasibility of a variety of operating conditions.

SUMMARY OF THE INVENTION

Aspects of the invention include a microreactor comprising: at least oneetched microchannel structure within a substrate having at least oneinlet and at least one outlet, at least one integrated heater, and atleast one catalyst material between the inlet and the outlet.

Another aspect of the invention includes a microreactor comprising: atop substrate and a bottom substrate such that at least one capillarymicrochannel is contained between the top substrate and the bottomsubstrate, the capillary microchannel having at least one inlet and atleast one outlet, a plurality of catalyst materials located between theinlet and the outlet, at least one porous membrane located at theoutlet, and at least one integrated heater.

Another aspect of the invention includes a method for forming a chemicalmicroreactor comprising: forming at least one capillary microchannelwithin a substrate having at least one inlet and at least one outlet,forming at least one porous membrane, imbedding the porous membrane withat least one catalyst material, integrating at least one heater into thechemical microreactor,

Interfacing the capillary microchannel with a liquid chemical reservoirat the inlet of the capillary microchannel, interfacing the capillarymicrochannel with the porous membrane at the outlet of the capillarymicrochannel, such that gas flow moves in a horizontal direction fromthe inlet through the microchannel and moves in a vertical directionfrom the microchannel through the outlet.

Another aspect of the invention includes a method of operating achemical microreactor comprising: delivering a fuel source from an inletthrough a microfluidic capillary that is packed with a catalyst materialto a porous membrane, heating the microfluidic capillary and the porousmembrane to a temperature between about 250° C. and about 650° C., andreforming the fuel source into hydrogen and a plurality of other gaseousmaterials while simultaneously passing at least the hydrogen through theporous membrane into at least one gas flow channel that is connected toat least one fuel cell.

Another aspect of the invention includes a method of operating achemical microreactor comprising: delivering a fuel source through afirst microfluidic capillary to a porous membrane that is imbedded witha catalyst material, heating the microfluidic capillary and the porousmembrane to a temperature between about 250° C. and about 650° C., andreforming the fuel source into hydrogen and a plurality of other gaseousmaterials while simultaneously passing at least the hydrogen through theporous membrane into at least one gas flow channel that is connected toat least one fuel cell.

Another aspect of the invention includes a method comprising: providingmeans for generating a hydrogen fuel from a liquid source, anddelivering the hydrogen fuel to a fuel cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an embodiment of a microreactor.

FIG. 1B shows a top view of the porous membrane structure portion of anembodiment of a microchannel.

FIG. 2A shows a cross-sectional view of an embodiment of a microreactorwith multiple microchannels.

FIG. 2B shows a cross-sectional view of the microchannel and resistiveheater portion of an embodiment of a microreactor.

FIG. 3 shows a cross-sectional view of the microchannel and resistiveheater portion of an embodiment of a microreactor with multiplemicrochannels.

FIG. 4A shows a cross-sectional view of the microchannel and resistiveheater portion of an embodiment of a microreactor.

FIG. 4B shows a cross-sectional view of the microchannel and resistiveheater portion of an embodiment of a microreactor with multiplemicrochannels.

FIG. 5 shows a cross-sectional view of an embodiment of a microreactorintegrated with a microcumbuster.

DETAILED DESCRIPTION

Referring to FIG. 1A, a chemical microreactor 2 comprises: a bottomsubstrate 4 a comprising silicon, glass or ceramic, a top substrate 4 bcomprising silicon, glass or ceramic, at least one capillarymicrochannel 6 having at least one inlet 8 for fuel and water and atleast one outlet 10 for gases, a liquid reservoir (not shown) containinga fuel source, at least one porous membrane 12, and at least oneintegrated heater 14 for heating the microchannel. Referring to FIG. 1B,a porous membrane support structure 13 comprising silicon, glass orceramic containing a plurality of porous membranes 12 is an effectivealternate embodiment to porous membrane 12 of FIG. 1A. Microreactor 2can further comprise a catalytic combustion microfluidic heat source(not shown) to heat the gases flowing through the microchannel andporous membrane(s).

Chemical microreactor 2 provides a means to generate hydrogen fuel fromliquid sources such as ammonia, methanol, and butane through steamreforming processes when mixed with the appropriate amount of water. Inan alternate embodiment to that shown in FIG. 1A, capillary microchannelinlet 8 mixes and delivers a fuel-water mixture from the liquidreservoir (not shown) through microchannel 6 and porous membrane 12.Porous membrane 12 can alternately be replaced with a porous membranesupport structure containing a plurality of porous membranes.

Referring to FIG. 2A, the fuel-water mixture can first be heated byresistive heaters in a “gassifier region” 15, i.e., the region where thefuel inlet connects to the microchannel, forming a fuel-steam gas. Thefuel-steam gas then flows through microchannel 6. The microchannel canbe packed with a catalyst material such as, platinum,platinum-ruthenium, nickel, palladium, copper, copper oxide, ceria, zincoxide, alumina, combinations thereof and alloys thereof. Resistiveheaters 14 can be positioned along the microchannel. Heatingmicrochannel 6 to a temperature between about 250° C. and about 650° C.by resistive heaters facilitates the occurrence of catalytic steamreforming reactions. The desired temperature depends upon the source offuel. For example, about 250° C. is an effective temperature if methanolis used, whereas ammonia requires a temperature closer to about 650° C.Microchannel 6 is formed in a configuration that allows adequate volumeand surface area for the fuel-steam gas to react as it flows throughmicrochannel 6 and porous membrane 12. Electrical connection pads 16provide current to resistive heaters 14. Although not shown, electricalpads 16 are connected to a power source. FIG. 2B is a cross-sectionalillustration of the embodiment depicted in FIG. 2A.

Two distinct embodiment styles are effective. The first embodimentemploys a packed catalyst capillary microchannel and at least one porousmembrane. In this embodiment, the primary purpose of the porous membraneis to prevent large particles or molecules flowing through themicrochannel to pass through the membrane. The porous membrane may ormay not contain catalyst materials.

The second embodiment style employs a porous membrane with a largesurface area or a porous membrane support structure containing aplurality of porous membranes having a large surface area in theaggregate, i.e., greater than about 1 m²/cm³. Surface areas on the orderof about 1 m²/cm³ to about 100 m²/cm³ are effective. In this embodiment,a catalyst material is imbedded within the porous membrane(s) and theprimary purpose of the porous membrane(s) is to facilitate theoccurrence of catalytic steam reforming reactions. Packed catalystcapillary microchannels may or may not be used with this embodimentstyle. This embodiment style can reduce the size and length requirementsof microchannel 6. For example, referring to FIGS. 1A and 1B,positioning porous membrane support structure 13 which contains aplurality of porous membranes 12 at outlet 10 of microchannel 6 providesa high surface area catalytic reaction. Minimizing the size of themicrochannel region in this manner makes it easier to heat and maintainmicrochannel 6 at the high temperatures required for the steam reformingreactions to occur, i.e., about 250° C. to about 650° C. Additionally,the porous membrane support structure 13 provides a flow interface withoutlet 10 and provides some restriction to gas flow resulting in aslight increase in the back-pressure of the microchannel region.

Hydrogen gas is generated by heating microchannel 6 and porous membrane12 to an appropriate temperature, i.e., about 250° C. to about 650° C.The fuel-steam source is reformed into gaseous byproducts, i.e.,hydrogen and subsequent byproducts, such as carbon monoxide and carbondioxide, as the molecules diffuse through the membrane and flow into afuel cell or other power source. Hydrogen is the component of the liquidfuel source that is converted into energy by a fuel cell. If chemicalmicroreactor 2 is used in concert with a fuel cell, the gaseousmolecules, after passing through the membrane structure, flow through atleast one other microchannel, i.e., a gas flow channel. The gas flowchannel is located at the exit side of catalytic membrane 12 and isconnected to the anode manifold of a fuel cell. Additional embodimentscan include the integration of a porous getter structure orpermaselective membrane material at the exit side of porous membrane 12to adsorb the product gases allowing only the hydrogen to diffusethrough to the fuel cell. It is beneficial to adsorb product gases ifthe presence of the additional byproducts will degrade the components ofthe fuel cell. Any fuel cell that uses hydrogen as a fuel source can beeffectively used with this invention. For example, effective fuel cellsinclude the micro-electro mechanical system based (MEMS-based) fuelcells discussed in U.S. patent application Ser. No. 09/241,159 by AlanJankowski and Jeffrey Morse which is hereby incorporated by reference.

A chemical microreactor can be constructed by using micromachining ofsilicon, glass, or ceramic materials, and wafer bonding. This method ofconstruction involves first forming the microchannel by etching apattern in the bottom surface of a substrate. For example, the patternmay be serpentine or straight. The depth of the microchannel isapproximately 200 μm, and penetrates only a fraction of the way throughthe total depth of the substrate, which can range in thickness fromabout 400 μm to about 600 μm. Referring to FIGS. 2A (top view) and 2B(cross-sectional view), resistive heaters 14 are formed on the topsurface of substrate 4 b and positioned above microchannel 6 in a mannerwhich optimizes the heat transfer from the heaters to the microchannels.The resistive heaters can also be formed on the top surface of substrate4 a, so that they are positioned adjacent to the surface of themicrochannel. Thus, the power input required to heat the fuel-water toproduct gases and complete the catalytic reaction as the gases flowthrough the channel is minimized.

Further embodiments facilitate a process referred to as counter-flowheat exchange. Such embodiments position the microchannels inconfigurations that permit the heat that is lost from the product gasesflowing through one microchannel to be transferred to gas flow streamsin adjacent microchannels. Such embodiments can include counterflow heatexchangers (not shown). The counterflow heat exchangers can be locatedin the following three areas and serve three different functions. First,counterflow heat exchangers can be located in the gassifier region toinitially heat the fuel water mixture. A second set of counterflow heatexchangers can be located in the area between the gassifier region andthe packed catalyst microchannel to add extra heat to the gas as itflows into the capillary microchannel. Finally, more counterflow heatexchangers can be located at the outlet of the porous membrane torecuperate any extra heat given off by the byproduct flow stream. Thehot gas outlet of catalytic microreactors integrated with a fuel cellconnect directly to the fuel cell anode manifold, and incorporate acounterflow heat exchanger at the fuel cell anode exhaust. Thatcounterflow heat exchanger transfers extra heat from the anode exhaustfrom the fuel cell back through the gassifier region and inlet flowstream to the catalytic microreactor.

The inlet port(s) 8 and porous membrane structure 13 are formed bypatterning and etching into the top surface of the substrate 4 a.Referring to FIG. 3, an inlet port 8 is approximately 1 mm in diameterand opens up to the entrance of microchannel 6. Separate inlets for fueland water may be formed, or a single inlet for premixed fuel-watermixtures (as shown in FIG. 3) may suffice. An array of vias 17 withdiameters ranging from 0.1-5.0 μm can be patterned and etched into aporous membrane support structure 13. The pores are straight, and gothrough to the end of microchannel 6 (for example, about 100 μm to about200 μm deep). Silicon can be etched using conventional plasma etch(Bosch process) techniques, laser etching, or photoinducedelectrochemical etching. Each etching technique will create an array ofvery straight, deep, narrow pores which extend to the microchannel,which is formed from the bottom side.

Another approach to forming a porous silicon membrane is to use anelectrochemical etch technique whereby hydrofluoric acid is used to etchpores in the silicon. The electrochemical etch creates a random porouslayer in the silicon. The pore sizes, for example, have diameters ofabout 0.1 μm to about 1.0 μm, and thicknesses on the order of about 60μm to about 200 μm.

A porous membrane support structure can be positioned at the outlet ofthe microchannel using a combination of thin film deposition, thick filmformation, and electrochemistry techniques. Referring to FIG. 4A, themembrane structure 13 may be a porous thick film structure comprisinganodic alumina, xerogel, or glass and is formed over an opening creatingvias 17 which are etched down to the microchannel 6 at the outlet end.FIG. 4B shows a multiple channel embodiment. In one example, a thickfilm membrane comprising xerogels is formed by depositing a solgelcoating of glass on the top surface of the substrate, and drying it insuch a way as to create random porosity through the film. For instance,a 30 minute bake at 120° C. to remove any remaining solvents is followedby a high temperature bake at 600-800° C. Others methods known to thosefamiliar with the art will also apply. The diameter of these pores mayrange in size from about 0.1 μm to about 1.0 μm, and the film can be upto about 100 μm thick.

In a second example, the membrane 13 is formed by bonding a porousalumina film about 50 μm thick to the top surface of the substrate 4 aover an opening leading to the microchannel 6. The porous alumina isformed by anodization of aluminum which creates arrays of narrow poresranging in diameter from about 0.02 μm to about 0.2 μm.

The porous thick film membrane structure has two primary purposes.First, it provides mechanical strength in the case where a pressuredifferential exists between the inlet 8 to microchannel and the outlet10 from the microchannel. Second, it provides a natural flow control ofthe gaseous reaction byproducts flowing through the porous membrane 12.The membrane structure can be controlled for the specific requirementsof the power source it is feeding. For example, the fuel, when fullyprocessed, in a 6 microliters/minute flow of a methanol:water (50:50)fuel mixture can provide approximately 500 milliwatts of electricalpower from a fuel cell at 50 percent efficiency if the microchannels andmicrofluidic system are designed to provide minimal pressure drops the 6microliters/minute flow rate.

Once the microchannels, porous membrane structures, resistive heaters,and counterflow heat exchangers are formed, the catalytic microreactoris completed by integrating the catalyst materials into the microchanneland porous membrane, then bonding a first substrate 4 a made of glass,silicon, or ceramic to a second substrate 4 b made of glass, silicon, orceramic.

The catalyst used may be platinum, platinum-ruthenium, nickel,palladium, copper, copper oxide, ceria, zinc oxide, alumina,combinations thereof, alloys thereof or other materials commonly used insteam reforming processes. Various coating methods are used to positionthe catalyst materials. For example, the catalyst materials can beimbedded within the membrane and the microchannel by thin filmdeposition techniques or they can be imbedded within the microchanneland porous membrane structure by ion exchange or solgel doping methods.These coating methods can be tailored to provide porous, high surfacearea coatings, thereby enhancing the reaction kinetics.

Other effective processes use small pellets or particles of a supportedcatalyst material, such as Copper/Zinc Oxide/Alumina, for example, whichare larger in diameter than the pore sizes of the porous membrane. Thiskind of catalyst material is commercially available, and is typicallyformed by imbedding the copper/zinc oxide materials in to a porousalumina support particle. Once formed, the catalyst particles can becolloidally suspended in a liquid solution. The colloidal solution canthen be injected through the microchannel. The porous membrane traps thecatalyst particles inside the microchannel. After some time, themicrochannel becomes filled with catalyst particles. This processcreates a packed catalyst microchannel that is porous enough for gasesto readily flow through and at the same time be exposed to a highsurface area of catalyst materials. This process can be used incombination with the catalyst coating methods described above, or byitself.

The membrane area and microchannel areas are made large enough to allowsufficient fuel flow for the power source requirements. In some cases,if resistive heaters require too much input electrical power to heat themicrochannels and porous membrane, exothermic combustion reactions maybe initiated. These exothermic combustion reactions may beself-sustaining and thus, do not require additional power.

Referring to FIG. 5, these self-sustaining exothermic combustionreactions can be accomplished by forming a microcombustor 20.Microcombuster 20 comprises a small microchannel 22 with a catalyst wireor electrode 23 (typically is a catalyst bed heater), which is separatefrom the capillary microchannel 6 and porous membrane 12, and at leastone electrical contact pad 30 connected to a power source (not shown).This microcombustor has a first inlet 24 for a fuel such as butane ormethanol, which is heated with a small resistive heater to form a gas,and a second inlet 26 for air or other oxygen-containing gaseousmixture. The fuel and air are mixed and flow over the catalyst wire orelectrode, which is heated by running a current through it similar to aresistor. The fuel/air mixture then ignites a combustion reaction whichgenerates heat, carbon dioxide and water. The heat is transferred to thecapillary microchannel and porous membrane and the carbon dioxide andwater flow to an outlet (not shown). Once ignited, the reaction issustained as long as fuel and air flow through inlets 24 and 26 withoutfurther current flowing through the catalyst wire 23 or filament. Theheat generated from the combustion reaction can be efficientlytransferred to the chemical microreactor and, if present, an integratedfuel cell, using the counterflow heat exchange process described above.The outlet gas stream from the microchannel combustor will be hot, andthis heat can be readily transferred through high surface areamicrochannels to adjacent cold gases flowing in opposite directions. Themicrochannel combustor can be formed using the same approaches describedabove for the chemical microreactor. In certain fuel cell embodiments,heat may be coupled between the steam reforming packed catalystmicrochannel and porous membrane and the fuel cell, thereby reducing thepower requirement to heat the fuel cell and make a very efficient powersource. The membrane material and porosity, catalyst deposition, andintegrated heater layout can be optimized to match a specific fuel, suchas methanol, or specific groups of fuels, such as ammonia, methanol andbutane.

Several microreactors can be integrated to allow processing of a varietyof liquid fuel components. Integrated microreactors which incorporateboth fuel cells and fuel reforming may be fabricated in parallel inorder to make them suitable for higher power applications ranging fromabout 10 Watts to about 50 Watts.

While particular operational sequences, materials, temperatures,parameters, and particular embodiments have been described and orillustrated, such are not intended to be limiting. Modifications andchanges may become apparent to those skilled in the art, and it isintended that the invention be limited only by the scope of the appendedclaims.

1. A method for forming a chemical microreactor comprising: forming atleast one capillary microchannel within a substrate having at least oneinlet and at least one outlet. forming at least one porous membrane.imbedding the porous membrane with at least one catalyst material,integrating at least one heater into the chemical microreactor,interfacing the capillary microchannel with a liquid chemical reservoirat the inlet of the capillary microchannel, interfacing the capillarymicrochannel with the porous membrane at the outlet of the capillarymicrochannel, such that gas flow moves in a horizontal direction fromthe inlet through the microchannel and moves in a vertical directionfrom the microchannel through the outlet, thereby producing a chemicalmicroreactor comprising: a plurality of separate reaction microchannelswithin a silicon substrate, each reaction microchannel within a siliconsubstrate, each reaction microchannel having at least one inlet and atleast one outlet, at least one of the reaction microchannels comprisinga steam reformer for a hydrogen-containing fuel having a reformingcatalyst material between the at least one inlet and the at least oneoutlet, and at least one other of the reaction microchannels comprisingan integrated catalytic microcombustion heater having at least oneheater catalyst material between the at least one inlet and the at leastone outlet, wherein at least one-of the at least one inlet and the atleast one outlet for each of the plurality of separate reactionmicrochannels is an additional non-reaction microchannel orientednon-parallel to the corresponding reaction microchannel, whereby a fullyintegrated silicon chemically heated steam reforming microreactor thatmaintains gas separation between the reformer and heater microchannelsis provided.
 2. The method of claim 1 further comprising: at least oneporous membrane located between said reformer inlet and said outlet. 3.The method of claim 1 wherein said catalyst material is selected fromthe group consisting of platinum, platinum-ruthenium, nickel, palladium,copper, copper oxide, ceria, zinc oxide, alumina, combinations thereofand alloys thereof.
 4. The method of claim 1, wherein the said reformeroutlet connects to a manifold of a fuel cell.
 5. The method of claim 1,wherein said at least one catalyst material located between said inletand said outlet is packed into said reformer microchannel.
 6. The methodof claim 2, wherein said at least one catalyst material located betweensaid inlet and said outlet are imbedded in said porous membrane in saidreformer microchannel.
 7. The method of claim 1, wherein said reformermicrochannel inlet connects to a liquid fuel reservoir.
 8. The method ofclaim 2, wherein said reformer microchannel is interfaced with saidporous membrane such that fuel flow moves in a horizontal direction fromsaid reformer microchannel inlet through said reformer microchannel andmoves in a vertical direction from said reformer microchannel throughsaid reformer microchannel outlet.
 9. The method of claim 1, whereinsaid heater is integrated at said inlet.
 10. The method of claim 1,wherein said heater is integrated along said reformer microchannel. 11.The method of claim 2, wherein said heater is integrated at said porousmembrane.
 12. The method of claim 2, wherein said porous membranecomprises a porous thick film selected from the group consisting ofporous silicon, anodic alumina, zerogel, glass and combinations thereof.13. The method of claim 2, wherein the catalyst material covers asurface area of the porous membrane measuring about 1 m²/cm³ or greater.14. The method of claim 1, wherein the microchannels support a fuel flowrate in the range of about 1 microliter/minute to about 600microliters/minute.
 15. The method of claim 2, further comprising: aporous getter structure located at the exit side of said porousmembrane.
 16. The method of claim 15, wherein the surface area per unitvolume of the getter structure is about 1 m²/cm³ or greater.
 17. Themethod of claim 1, wherein said microreactor is configured to processmore than one type of liquid fuel component into hydrogen fuel.